Low-grade metamorphic rocks of the Tethys subduction–collision zone in the Medvednica Mt. (NW Croatia)

Abstract

Igor Broska) Abstract: The low-grade metasedimentary rocks of Medvednica Mt. (Northwest Croatia) are associated with ortho-greenschists (metabasalts). The Lower to Upper Triassic age of the protolith of the low-grade metamorphic rocks belonging to the Stari Potok, Adolfovac, Tusti Breg, Bliznec, Mrzljak, and Sljeme lithostratigraphic units is determined on the basis of conodonts and field relationships. The metamorphic age of the Triassic metasedimentary rocks from the Risnjak lithostratigraphic units is assumed to be Jurassic. Petrographic and geochemical analyses of ortho-greenschists point to a tholeiite basic magmatic protolith rock having T-MORB (transitional midocean-ridge basalt) characteristics. The ortho-greenschists of the Sljeme lithostratigraphic unit underwent retrograde metamorphism and show metamorphic zoning from the epidote–amphibolite facies to the lower parts of the greenschist facies. However, the greenschists of the Tusti Breg and Bliznec units are characterized by prograde metamorphism ranging from lower to higher greenschist facies. This metamorphism is associated with Jurassic subduction (165–150 Ma) and closure of the Western Tethys. Retrograde metamorphism took place during the collision of the Adriatic and Eurasian plates in the Lower Cretaceous This study represents the new geochemical and petrological data on the Triassic protolith of the low-grade metamorphic rocks of Mt. Medvednica (NW Croatia). The aim of the paper is to determine the chronostratigraphic age of the protoliths of the studied metamorphic rocks, including the time and grade of metamorphism, as well as define the mineralogical, petrological, and geochemical characteristics of ortho-greenschists of Mt. Medvednica and elucidate the geotectonic setting of their formation. On the basis of the results, the protolithic age of the metamorphic rocks was reinterpreted, and thus metamorphism was associated with Alpine orogeny during the closure of the Western Tethys. Additionally, in order to examine the hypothesis of the single Tethyan ocean area during the Mesozoic the analysed ortho-greenschists and their protoliths from the metamorphic complex of Mt. Medvednica were compared with analogous rocks in ophiolite mélanges from the neighbouring magmatic–sedi-mentary complexes (ophiolite mélanges) of the Zagorje–Mid-Transdanubian Zone (ZMTDZ). The analysis of geochemical affinities and geological characteristics of analogous rocks from the will contribute better knowledge the during the